The advent of applications such as network computing, multimedia, video conferencing, and real-time imaging require data rates ranging into the gigabits-per-second. The demand for such high rates has led the industry to combine a standardized wide band network (SONET) with the simplicity of an efficient network that uses fixed-length 53-byte-wide asynchronous transfer-mode (ATM) cells. In 1992 ATM was chosen by the CCITT (Consultative Committee for International Telephony and Telegraphy) as the transport technology for the huge variety of services to be offered by the Broadband Integrated Services Digital Network (B-ISDN). However, it has been recognized that ATM is equally well-suited for use in the local area network. An ATM cell consists of 53 octets or bytes with a 5 byte cell header containing control bits and a 48 octet or byte cell payload which contains the data bits. In order to interface with a standardized wide band network such as Synchronous Optical Network (SONET) appropriate interfaces to transfer from one system to the other are required.
In order to integrate all of the functions of a SONET/SDH-3c ATM physical layer interface into a single device and at the same time be applicable to local and wide area networking applications a number of criterion have to be met. First one requires a fully compliant SONET/SDH framer. Here the term SDH refers to CCITT's synchronous digital hierarchy and STS-3c refers to a data transmission rate of 155.52 megabits-per-second (Mbits/s). Being a multiple of 3 times the STS-1 base rate of 51.84 Mbits/s in accordance with the SONET family of interface rates. The SONET STS-3c frame structure consists of 9 rows of bytes with each row having 9 bytes of transport overhead and 261 columns of 9 bytes each with one of the columns having control bits defining path overhead while the remaining columns are payload. The framer takes ATM cells and puts them into a synchronous series of SONET FRAMES.
A second requirement of an interface device is an ATM cell processor to perform cell delineation and null cell insertion/filtering. Since many of the services delivered by ATM are by definition asynchronous, they are characterized by a non-continuous cell stream. Thus, cell rate decoupling transforms a non-continuous cell stream into a continuous stream by inserting idle or null cells (cells containing no payload) during idle periods in the assigned cell stream. By making the cell rate continuous, it is necessary only to synchronize with the incoming cells in order to place the ATM cells in their assigned locations in a frame.
The conventional system of processing data byte-by-byte leads to long waiting times in order to transfer a whole cell. This is particularly important when multiple sources of ATM signals are being received. For example, the receiving system would be tied up waiting for all bytes from a given cell to arrive, even though bytes from other cells were also arriving. It is important for a user network interface to overcome such inefficiencies.
A third requirement is a line side interface to support both serial input/output at 155 Mbits/s and parallel input/output at 19.44 Mbyte/s. The receive interface portion of the line side interface must be a low speed generic byte wide interface for interconnection with an upstream serial-to-parallel converter, or with an upstream byte interleaved demultiplexer. When operating with a serial to parallel converter, the interface must provide scrambled data and a 19.44 MHz clock. When operating with the byte interleaved multiplexer, the transmit interface provides an STS-3c/STM-1 stream which is multiplexed to a higher rate stream such as an STS-12/STM-4. In the latter case no scrambling of the data takes place.
Finally, there must be an overhead expansion port to access all SONET/SDH overhead bytes to allow external processing for full SONET/SDH network-network interface (NNI) compliance. By allowing external monitoring and/or control of the overhead bytes additional SONET/SDH features can be implemented externally when required for specific applications.
For SONET/SDH systems, prior solutions had to be implemented with multiple devices or with discrete logic. One known SONET ATM interface device implemented a subset of the SONET/SDH protocol sufficient for demonstration purposes but not for use in actual commercial applications. It did not incorporate cell delineation or a drop side FIFO based interface supporting cell rate decoupling or provide an overhead expansion port to allow feature set enhancement.